The invention relates to a method of manufacturing a discharge lamp provided with a discharge vessel which encloses a discharge space with a ceramic wall, said discharge vessel being obtained through sintering of a body formed in a sludge molding process, which sludge molding process comprises the following steps:
injecting a porous outer mold with sludge,
deposition of sludge particles against the injected outer mold, and
removing excess sludge, removing the outer mold, and pre-firing of the molded body.
The invention also relates to a lamp provided with a discharge vessel with a ceramic wall.
Such a method is known from EP 0926106. The sludge is a liquid suspension of sludge particles in a suspension liquid. Water is generally used as the suspension liquid. A sludge molding process is understood to involve, in the context of the present description and claims, a process in which a closed space surrounded by the porous outer mold is filled with sludge, the sludge particles are deposited against the mold wall owing to a removal of suspension liquid, and the remaining, still liquid sludge is decanted. The removal of suspension liquid may take place here both by means of capillary absorption of the suspension liquid by the outer mold and by means of a forced escape of suspension liquid through an applied pressure difference between the enclosed space and the surroundings of the outer mold.
The method allows the formation of a discharge vessel in which a portion designed for enclosing a discharge space and portions designed for accommodating electrical lead-through members are formed as one integral whole. The risk of a sintering seam, which forms the connection between different portions of the discharge vessel, becoming leaky is eliminated thereby.
In practical lamps, the discharge vessel is provided with a discharge space portion which is enclosed by the ceramic wall and has projecting closures at mutually opposed ends, which closures are formed as elongate tubular plugs each with a free end through which the electrical lead-through member is passed to an electrode positioned in the discharge space. Each plug is closed at its free end by means of a suitable melting glass or melting ceramic. The melting glass or melting ceramic also provides an adhesion between the plug and the associated lead-through element.
A disadvantage of the known method is that it leads to the formation of practical lamps with a comparatively great external diameter of the plugs. This may give rise to an undesirable heat balance of the manufactured lamp. A further disadvantage is that an after-treatment of the plug is often found to be necessary for realizing a suitable lead-through opening. The after-treatment consists, for example, in reaming of the plug of the molded discharge vessel, possibly followed by a polishing treatment. After-treatments are disadvantageous because they make the lamp manufacturing process more complicated and also because they increase the risk of production wastage.
The invention has for its object to provide a means whereby the disadvantages mentioned above are effectively counteracted.
According to the invention, a method of the kind mentioned in the opening paragraph is for this purpose characterized in that the sludge is injected into the outer mold by means of a hollow needle which extends to inside the outer mold. The use of a hollow needle as an injection element extending to inside the hollow mold during injection has the advantage that it acts as an inner mold at the same time. An internal surface portion of the object to be formed, i.e. the discharge vessel to be formed, is defined in a very simple and also very reliable manner through a suitable choice of the external diameter of the hollow needle. It was surprisingly found by the inventors that a projecting plug created in this way and designed to act as a lead-through portion of the discharge vessel does not require any after-treatment after sintering. A further advantage is that in this way the wall thickness of the lead-through portion is found to be well controllable, with the result that the choice of dimensions of the lead-through portion can be largely independent of the choice of dimensions of the discharge space portion of the discharge vessel. Thus, in an advantageous embodiment of the lamp obtained by the method, the discharge space portion of the discharge vessel has a wall thickness D and the lead-through portion has a wall thickness d complying with the relation: d/Dxe2x89xa61.3. Preferably, the requirement is also met that the lead-through portion has an external diameter du and the discharge portion a greatest external diameter DU complying with the relation: du less than 1.5DU, and for a lamp with a power rating of at most 150 W preferably du less than DU.
Preferably, the outer mold has a longitudinal axis, and a hollow needle with a needle axis is present at both ends of the mold, said needle axis coinciding with said longitudinal axis, and the sludge is injected trough said needle in the method according to the invention. The use of this method is highly suitable for the manufacture of a discharge vessel with two diametrically opposed electrodes which are each connected to a lead-through member.
The invention can be used to particular advantage if a discharge vessel provided with a projecting plug is manufactured by the method. It is thus possible in a reliable but simple manner to form a projecting plug which is integral with the rest of the discharge vessel and whose dimensions comply with strict dimensional requirements. Preferably, the hollow needle extends inside the outer mold over at least the length of the projecting plug to be formed. It may be advantageous in this respect that the length over which the hollow needle extends inside the outer mold is substantially equal to the length of the projecting plug plus the wall thickness of the discharge vessel at the area of the discharge space.
CaSO4 (plaster of Paris) is a very suitable mold material which shows a very favorable unmolding behavior. The use of plaster of Paris may lead to some contamination of the molded body with Ca. This may influence the crystal structure of the sintered body under certain circumstances. If such an influence is undesirable, it is preferable for the outer mold to be formed from a material akin to that of the sludge particles at least at its surface against which the sludge particles are deposited during the formation of the vessel. It can be prevented thereby that contamination of the body formed in the sludge molding process has an undesirable influence on the sintering process. A suitable possibility for this is, for example, that the material of the outer mold is of the same kind as that of the ceramic wall to be formed, such as, for example, Al2O3. Often a so-called sintering dopant, for example MgO, is added to the sludge mass for obtaining a controlled crystal growth of the body formed by the sludge molding process during subsequent sintering. A suitable material for the outer mold surface is a material akin to the material of the sintering dopant. If the sludge contains MgO, a very suitable material is, for example, MgCO3. The MgCO3 has the favorable property that it has a solubility product in water which is substantially equal to that of CaSO4.